Use of petroleum is expected to increase, despite diminishing world reserves of high quality crude oils. The sulfur oxides generated from the combustion of petroleum products may play a major role in the formation of acid deposition ("acid rain") which produce lasting detrimental effects on aquatic as well as terrestrial ecosystems. The demand for low sulfur petroleum has been intensified by increasingly stringent regulatory standards for reduced levels of sulfur oxide in atmospheric emissions. For compliance with the regulatory standards, fuels containing high sulfur content must be subjected to either pre- or post-combustion desulfurization. Processes for desulfurization of oil currently in use, both chemical and physical, still remain inadequate for coping with the problem. Desulfurization of heavy oil by hydrogenation or by other chemical procedures requires either special catalysts, extremely high temperatures, pressures or combinations thereof, making the process very expensive. Heavy metals in petroleum limit the use of hydrodesulfurization processes as they poison the catalysts used in such processes. Since both sulfur compounds and heavy metals are concentrated during refinery processing, sulfur removal from residual fuel oil is even more difficult. These disadvantages of conventional desulfurization processes and increased concern with the acid rain problem have stimulated interest in microbial sulfur transformations and the technology for microbial precombustion desulfurization.
The ubiquity of aromatic-thiophene derivatives in virtually all crude oils has led to the use of dibenzothiophene (DBT) as a model compound in investigation of crude oil desulfurization. Aerobic bacteria, such as Pseudomonas sp., Beijerinkia sp., Bacillus sulfasportare, have been used in attempts to remove organic sulfur compounds from petroleum and its products as water soluble forms (Malik, K. A. 1978, Process Biochem., 13 (9), 10). But, this process was found not to be economically feasible, and it was thought that air supply to the system for the oxidation of sulfur compounds by aerobic bacteria was dangerous.
The possibility of using anaerobic bacteria for the removal of sulfur compounds has been reported. Davis and Updegraff (1954. Bacteriol. Rev., 18, 215) attempted the microbial desulfurization of crude oil by certain sulfate reducing bacteria, but not successfully. Zobell in his patent (1953, U.S. Pat. No. 2,641,564) claimed that sulfate reducing bacteria with high hydrogenase activities can reduce organic sulfur in petroleum to hydrogen sulfide. The reactions were believed to be generally as follows: EQU R-S-R'+H.sub.2 .fwdarw.RSH+R'H (or RH+R'SH) EQU R-S-R'+2H.sub.2 .fwdarw.RH+H.sub.2 S+R'H
Kurita et al (1971, J. Gen. Appl. Microbiol., 17, 185) showed that bacterial cultures which produce H.sub.2 S from organic sulfur compounds in petroleum have been isolated under anaerobic condition. The isolates were also found to produce H.sub.2 S from hydrogenated residue oil, crude oil, and asphaltene. Methyl viologen as an electron mediator was essential for the reaction. Kohler et al (1984, Zbl. Mikrobiol., 139, 239) reported that a selected Desulfovibrio strain utilizing molecular hydrogen can reduce nine sulfur compounds found in petroleum to H.sub.2 S, and that the degree of desulfurization achieved related to growth conditions and hydrogenase activity of the cultures. Eckart et al (1986, Zbl. Mikrobiol., 141, 291) used the mixed cultures from different sediments for anaerobic desulfurization of Romashkino petroleum. Main components of these mixed cultures were Desulfovibrio sp. and concomitant strains were found to be micrococci, bacilli, clostridia. The microbial desulfurization processes utilizing anaerobic bacteria described above also have a drawback. That is, hydrogen gas has to be supplied to the system for the reductive desulfurization of organic sulfur compounds in petroleum by sulfate reducing bacteria.
Therefore, utilization of electrochemical energy instead of hydrogen gas as a source of reducing equivalent would be very desirable for the convenient desulfurization of petroleum and its products.